1. Field of the Invention
The present invention relates to systems and methods for distributed control of the inductance and capacitive loading of high-voltage power lines and specifically relates to such systems using an injection transformer mounted on a transmission tower.
2. Prior Art
One of the requirements of improving the efficiency of the power grid is the removal of transmission bottlenecks related to active power flow control. The need is to control where and how the real power flow is achieved on the grid. Congested networks limit the system reliability and increase the cost of power delivery across the power grid. To improve the power flow through-put of the grid, it is necessary to be able to adjust the power flowing along any of the wires. Unbalanced lines produce uncontrolled loop currents, overloading the lines with increased losses. Active power flow control provides the best solution for this problem by altering the line impedances and changing the angle of the voltage on the respective line, thereby controlling power flow. Active power flow control using impedance injection (both capacitive and inductive) with centralized control at network level has been proposed in the past but the complexity and cost of such systems have prevented implementation. Most of the grid control capabilities by injecting impedance are still ground based, installed at substations with switch-able inductive and capacitive loads that have the associated requirement for high-voltage insulation and high-current switching capabilities. Being at the substations, they are able to use cooling methods that include oil cooling etc. with less weight limitations and less limitations of the profile of the units used. There is consensus that future power grids will need to be smart and aware, fault tolerant and self-healing, dynamically and statically controllable, and asset and energy efficient. It has also been understood that distributed active impedance injection units that are intelligent and self aware will be able provide the needed distributed control of the line impedance if such can be effectively implemented with high reliability. Such a system implementation can provide the solution to this dilemma and improve the system power grid efficiency substantially.
At present there are few solutions for distributed control of the power grid, which have been proven effective and reliable. One such system is the Power Line Guardian™ commercial product from the assignee of the current application. FIG. 1 shows the block schematic 102 of the current static distributed control capability. These static distributed control modules 110 and 100 are directly attached to high-voltage-transmission line segments 108 to provide distributed control of the transmitted power from the generator 104 to the distribution stations 106. These solutions provide a limited amount of control success by adding inductive load to the lines by switching the inductance load in and out of circuit, and are described in U.S. Pat. No. 7,105,952 and U.S. Pat. No. 7,835,128 currently licensed to the assignee of the current application. PCT Publication No. WO2014/099876, owned by the assignee of the current application, discusses the physical installation on the power line.
The distributed module 100 is a distributed series reactor (DSR) that impresses a static inductive load using a transformer with a single turn primary winding (the HV-line section) with a single multi-turn secondary winding by having a pre-defined inductance switched on and off to impress the inductance on the HV-transmission line. The DSR 100 allows a passive, switch-able distributed inductance to be gradually inserted into a conductor 108, thus effectively increasing the line impedance and causing current to direct into other lines that have additional capacity. A distributed series impedance device such as the DSR 100 is clamped around the conductor 108 using a single turn transformer (STT).
FIGS. 2 and 2A and 2B show embodiments of a passive impedance injection module DSR 100. The HV transmission line 108 is incorporated into the module as the primary winding by adding two split-core sections 132 that are assembled around the HV transmission line 108. The core sections 132 are attached to the HV transmission line 108 with an air gap 138 separating the sections after assembly. The air gap 138 is used to set a maximum value of fixed inductive impedance that is to be injected on the HV line via the primary winding. Secondary winding 134 and 136 encircles the two split-core sections 132 and enabled the bypass switch 122 to short out the secondary winding and prevent injection of inductive impedance on to the a HV transmission line 108 and also provide protection to the secondary circuits when power surges occur on the HV transmission line. The split core sections 132 and the windings 134 and 136 comprise the single-turn transformer (STT) 120. A power supply module 128 derives power from the secondary windings 134&136 of the STT 120 via a series connected transformer 126. The power supply 128 provides power to a controller 130. The controller 130 monitors the line current via the secondary current of the STT 120, and turns the bypass switch 122 off when the line current reaches and exceeds a predetermined level. With the contact switch 122 open, a thyristor 124 may be used to control the injected inductive impedance to a value up to the maximum set by the air gap 138 of DSR 100.
Distributed active impedance injection modules on high voltage transmission lines have been proposed in the past. U.S. Pat. No. 7,105,952 of Divan et al. licensed to the applicant entity is an example of such. FIG. 3 shows an exemplary schematic of an active distributed impedance injection module 300. These modules 300 are expected to be installed in the same location on the HV power line as the passive impedance injection modules (or “DSR” 100) shown FIG. 1. The active impedance injection module 300 does not perform the same functions. In fact the active impedance injection module 300 does not have a gapped core 132 of FIG. 2B that provides the fixed inductive impedance. Instead the inductive or capacitive impedance is generated using the converter 305 based on the sensed HV transmission line 108 current. Sampling the secondary current by the series-connected secondary transformer 302 does the sensing of the magnitude of the line current. The sensing and power supply block 303 connected to the secondary transformer 302 extracts the HV transmission line current information and feeds the controller 306. The controller based on the received input provides the necessary commands to the converter 305 to generate the required inductive or capacitive impedance to adjust the line impedance. The value of the impedance in this case is not fixed but varies according to the status of the measured current on the HV transmission line. Hence the system using spatially distributed active impedance injection modules 300 provides for a much smoother and efficient method for balancing the grid.
In practice the active impedance injection modules 300s have not been practical due to reasons of cost and reliability. In order to inject the needed impedances on to the HV transmission line for providing reasonable line balancing there is a need to generate a significant amount of power in the converter circuits. This has required the active impedance injection modules 300 to use specialized devices with adequate voltages and currents ratings.
The failure of a module in a spatially distributed inductive impedance injection line balancing system using DSR 100 modules inserts a fixed inductive impedance set by the “air gap” 138 or substantially zero impedance on to the line. Failure of a few modules out of a large number distributed over the HV transmission line does not mandate the immediate shutdown of the line. The repairs or replacement of the failed modules can be undertaken at a time when the line can be brought down with minimum impact on the power flow on the grid. For utilities to implement distributed active line balancing, the individual modules must be extremely reliable. They also have to be cost effective to be accepted by the Utilities.
Power transmission line balancing circuits have been limited to the use of delayed-acting heavy-duty fully-insulated oil-cooled inductive and capacitive impedance injectors or phase-shifting transformers prone to single-point failures, located at substations where repairs of these failed units can be handled with out major impact on power transfer over the grid.
A STT clamped to the HV-transmission line has limited influence on the power flow. A multi-turn transformer (MTT) would have many times the influence of the STT but requires cutting the HV-transmission line to install. The HV-transmission line typically hangs from a suspension tower which applies a vertical force to support the weight of the HV-transmission line. The HV-transmission line has significant horizontal tension and a break in the HV-transmission line can exert sufficient force to cause the suspension towers to topple over. Installing, maintaining and replacing a MTT that connects to the two ends of a cut in the HV-transmission line would be difficult, expensive and potentially dangerous. A STT clamped to the HV-transmission line has a weight limitation that inhibits its influence. The influence of a transformer depends on the weight of the transformer. The clamped STT must be light enough to avoid adding excessive tension to HV-transmission line and needs to be stable in extreme weather conditions such as high winds.
The HV-power grids would benefit significantly if the distributed series reactors could exert greater influence by, for example, using transformers with more weight.